aquila-enu It is natural that many people wish to use proprietary closed-source software without compromising privacy, and to, because of that, look for recipes to turn potentially privacy-invasive software into privacy-respecting software. But that is often not possible.
It is a fact that on Android applications can communicate with each other via IPC. Android was designed that way, so that apps can cooperate with each other to get things done that users want. An example of this is sharing a photo: an app with a photo to share contacts apps that are capable of accepting the photo.
It is also a fact that Android apps can be, and often are, written to temporarily work offline if a network connection is unavailable. This includes building a "todo list" of communication actions to take when a network connection is later available. One example of this would be when a user edits a local copy of a file when there is no network connection; the app can remember to transmit the file to a cloud server when a network connection is available later.
All in all, it is absolutely technically possible for a photo app (such as Photos) that has no network connection to share a photo with a framework app (such as Play Services) that does have a network connection, and for the framework app to send the photo to some server. This is not necessarily nefarious! One reason for writing a framework app is so that multiple apps can offload code and management cycles to the framework app. In some cases it is possible that the user-facing app (such as Photos) would be designed to never access the network on its own and always delegate network traffic to a framework app.
It is also possible for a photo app that has no network connection to share a photo with a framework app that also doesn't have a network connection now, and for the framework app to send the photo to some server later when it does have a network connection.
So:
- It would be nice if removing the Network permission from a photo app caused the photo app to respect the user's privacy, but that is not necessarily the case,
- It would be nice if granting an app the Network permission only every now and then to download plugins meant that the app didn't use the period of network access to transmit information, but that is not necessarily the case,
- So far it does not appear that anybody has documented Google Photos using Play Services to upload photos,
- But that is absolutely technically possible.
There is no conflict between the two sources you cited. The unwanted behavior is both absolutely technically possible and not proven. In other words, as you quoted, "We know they can, not that they do".
It might make sense to investigate photo apps with privacy policies you find reasonable, including perhaps open-source photo apps, including perhaps donating to authors of open-source photo apps to support improvements in those apps.